In a reciprocating internal combustion engine, intake and exhaust valves are urged towards their closed positions by valve springs consisting of helical compression springs, and are closed and opened by transmitting drive forces from cams which are rotated in synchronism with the rotation of the engine to the stem ends of the valves via rocker arms. Such helical springs are provided with closed and ground ends so that they may stand upright and no localized stress may be generated in the areas of contact between the helical compression springs and spring retainers supporting them.
The output of an engine can be increased by increasing its maximum rotational speed, but it is limited by the occurrence of leaping and surging of the valves due to the resonance of the valve spring system. Japanese patent publication No. 47-22363 discloses a technology for elevating the resonance point of a valve spring system by giving a non-linear property to the valve springs through uneven distribution of the winding pitch.
As an alternate approach to elevating the resonance point of the valve spring system, there is a possibility of manufacturing valve springs with such materials as titanium alloys which have high mechanical strengths for their weights and high modulii of transverse elasticity. According to the experiments conducted by the inventors, it was discovered that the resonance frequency of the valve spring system can be increased by using titanium alloys and increasing the spring constant of the spring while reducing its weight.
However, it was also discovered that after being used in an internal combustion engine which was operated at high speed over an extended time period, the valve springs, in particular, their end portions became excessively worn and through increase in the stress acting in those parts the valve springs tended to be prematurely and permanently deformed and, in some cases, actually broke. Presumably, such premature wears were caused by the mutual striking of different parts of the coil wire in the end portions of the helical springs.